In internal combustion engines, combustion can be effectively controlled by measuring an in-cylinder pressure and by detecting a peak in-cylinder pressure and an ignition timing, on the basis of the measured pressure, within one combustion cycle consisting of intake, compression, expansion and exhaust so as to feedback-control an air-fuel ratio and a fuel-injection timing of each cylinder. In order to further improve this effect of optimizing the air-fuel ratio and the fuel-injection timing by feedback controlling of each cylinder, their in-cylinder pressures have to be measured with higher precision. To this end, JP Patent Application Publication No. 4-314951 A (1992), for example, discloses a correction technique using, as a reference pressure, an intake pressure and an exhaust pressure detected by a piezo-electric device type pressure sensor detecting an in-cylinder pressure.    Patent Document 1: JP Patent Application Publication No. 4-314951 A (1992)